Hibernation | Article about hibernation by The Free Dictionary Hibernation | Article about hibernation by The Free Dictionary Also found in: Dictionary , Thesaurus , Medical , Wikipedia . hibernation (hī'bərnā`shən) [Lat.,= wintering], practice, among certain animals, of spending part of the cold season in a more or less dormant state, apparently as protection from cold when normal body temperature cannot be maintained and food is scarce. Hibernating animals are able to store enough food in their bodies to carry them over until food is again obtainable. They do not grow during hibernation, and all body activities are reduced to a minimum: there may be as few as one or two heartbeats a minute. Cold-blooded animals (e.g., insects, reptiles, amphibians, and fish) must hibernate if they live in environments where the temperature—and hence their own body temperature—drops below freezing.

Some insects pass their larval stage in a state of hibernation; in such cases hibernation is closely associated with the reproductive cycle (see larva larva, independent, immature animal that undergoes a profound change, or metamorphosis, to assume the typical adult form. Larvae occur in almost all of the animal phyla; because most are tiny or microscopic, they are rarely seen. They play diverse roles in the lives of animals. ..... Click the link for more information. ; pupa pupa , name for the third stage in the life of an insect that undergoes complete metamorphosis, i.e., develops from the egg through the larva and the pupa stages to the adult. ..... Click the link for more information. ). However, most warm-blooded animals, i.e., birds and mammals, can survive freezing environments because their metabolism controls their body temperatures. Many hibernating animals seek insulation from excessive cold; bears and bats retire to caves, and frogs and fish bury themselves in pond bottoms below the frost line. Analogous to hibernation is aestivation, a dormant period of escape from heat and drought.

Other methods of avoiding excessively high or low temperatures and destructive increases or decreases in the water supply are encystment and ensuing dormancy, e.g., in plant seeds and bacteria, and migration. Some animals, such as rabbits, raccoons, and squirrels, store food against scarcity and spend cold periods asleep in their burrows, though they may emerge on warm days. Hibernation A term generally applied to a condition of dormancy and torpor found in cold-blooded (poikilotherm) vertebrates and invertebrates. (The term is also applied to relatively few species of mammals and birds, which are warm-blooded vertebrates.) This rather universal phenomenon can be readily seen when body temperatures of poikilotherm animals drop in a parallel relation to ambient environmental temperatures. Poikilotherm animals Hibernation occurs with exposure to low temperatures and, under normal conditions, occurs principally during winter seasons when there are lengthy periods of low environmental temperatures. A related form of dormancy is known as estivation. Many animals estivate when they are exposed to prolonged periods of drought or during hot, dry summers.

For all practical purposes, hibernation and estivation in animals are indistinguishable, except for the nature of the stimulus, which is either cold or an arid environment. There is no complete list of animals that hibernate; however, many examples can be found among the poikilotherms, both vertebrate and invertebrate. The poikilotherms are sometimes referred to as ectothermic, because their body temperatures are not internally regulated but follow the rise and fall of environmental temperatures. During hibernation and winter torpor, body temperatures reflect the environmental temperature, often to within a fraction of a degree. Among the classic examples of hibernators or estivators are reptiles, amphibians, and fishes among the vertebrates, and insects, mollusks, and many other invertebrates. For many ectothermic vertebrates (fishes, amphibians, and reptiles) the ability to avoid seasonal and periodic environmental rigors by entering a state of metabolic inactivity is a crucial element in their survival.

Specifically, winter dormancy and summer estivation—the usual context in which these terms are applied to ectotherms—permit these animals to survive and flourish, first, by reducing the impact of seasonal extremes and, second, by significantly lowering the ectotherm's energetic costs during times that would not be favorable for activity (that is, when food is available). Many terrestrial reptiles, such as lizards, snakes, and turtles, become dormant and hibernate by burrowing in crevices under rocks, logs, and in the ground below the frost line. Terraqueous turtles also become cold-torpid and may often be found completely submerged in mud and in ponds under ice. Since the hibernating reptile is subject to the caprices of duration of seasonal low temperature, there is no well-defined period of dormancy. The period of hibernation may often be related to latitudinal positions as evidenced by the turtle family Emydae. Species that inhabit the northern climes will hibernate longer than their southern relatives, thus showing hibernation periods which are proportional to the length of the winter period. Hibernating reptiles show a loss of appetite and discontinue the ingestion of food.

Although the metabolic rate is reduced as much as 95% in hibernating turtles, there is some utilization of stored food products. There are two principal types of reserve food: lipids and glycogen, the animal starch, which is less stable and more rapidly used than fats. Glycogen is generally localized in tissues such as liver and muscle. There is evidence that these reserve foods are selectively utilized. In hibernating turtles, the tissue glycogen is used during the initial days and weeks of hibernation; later, the lipids are utilized. A major hazard to hibernating poikilotherms is death from freezing; ice crystals form in free protoplasmic water and ultimately destroy the cells and tissues, causing the death of the animal. Frogs, salamanders, and turtles are able to survive, despite the reduction in body temperatures to about 32 to 31°F (0 to -1°C). As winter approaches, the water content of the tissues becomes reduced and the blood more concentrated. Hibernation in fishes does not occur. Many fishes do, however, spend much of the winter in a state of quiescence while partially frozen in mud and ice.

The phenomenon of estivation is best known in the dipnoans, that is, the lungfishes. These fishes are restricted to tropical regions marked by repeated seasons of drought. They survive the dry seasons by becoming dormant and torpid. The lungfishes are among the more primitive air-breathing animals possessing a lung which utilizes atmospheric oxygen. This lung becomes the primary organ of respiration during the torpidity of estivation. In general, the lungfishes follow a similar behavioral pattern as the dry seasons approach. Protopterus, for example, burrows in the bottom mud as the water begins to diminish during the dry season. A lifeline of air is provided by the tunnel from the burrow to the surface. In preparation for estivation, Protopterus secretes a slimy mucus around itself which hardens in a tight cocoonlike chamber, preventing the desiccation of the fish. There is but one opening, formed around the mouth. Thus the air from the tunnel enters the mouth and passes to the lung apparatus. At the termination of the dry season, water slowly enters the burrow, softens the contents, and awakens the lungfish.

The metabolism of the lungfish is at a low ebb during estivation, with the energy for its modest life processes provided by the utilization of tissue protein. In some snails estivation may be extended for years at a time, and among the insects and spiders the period of hibernation becomes intimately associated with a phase in the life cycle. During the winter months and during a hot dry summer, the soil contains a remarkable variety of torpid invertebrates, for example, earthworms, snails and slugs, nematodes, insects and spiders, grubs, larvae, and pupae of many insects, egg cases, and cocoons. Insects overwinter, for the most part, in the egg or larval stage of metamorphosis. Hibernation frequently becomes integrated with the diapause, or arrested development, of the egg or larva which occurs during the winter. The familiar cocoon of the butterfly is the hibernaculum of the larva and pupa. See Insect physiology The phenomenon of encystment is commonplace in the protozoa, or single-celled animals.

Encystment is remarkably similar to estivation and hibernation, and an encysted protozoon is extremely quiescent and almost nonmetabolizing. See Protozoa The hibernacula of poikilotherm vertebrates and invertebrates are as varied as the animals themselves (see illustration). The minute cysts in protozoa, the cocoon and egg case of insects and spiders, the burrows and crevices of reptiles, and the dried mucous case of the lungfish, in all instances, protect the animal from evaporation or desiccation and freezing. Hibernacula of various cold-blooded vertebrates Warm-blooded vertebrates Many mammals and some birds spend at least part of the winter in hiding, but remain no more drowsy than in normal sleep. On the other hand, some mammals undergo a profound decrease in metabolic rate and physiological function during the winter, with a body temperature near 32°F (0°C). This condition, sometimes known as deep hibernation, is the only state in which the warm-blooded vertebrate, with its complex mechanisms for temperature control, abandons its warm-blooded state and chills to the temperature of the environment.

Between the drowsy condition and deep hibernation are gradations about which little is known. The bear, skunk, raccoon, and badger are animals which become drowsy in winter. Although usually considered the typical hibernator, the bear's body temperature does not drop more than a few degrees. The deep hibernators are confined to five orders of mammals: the marsupials, the Chiroptera or bats, the insectivores, the rodents, and, probably, the primates. Most, if not all, of the insect-eating bats of temperate climates not only hibernate in the winter, but also drop their body temperature when they roost and sleep. The advantage of this for a small mammal with a disproportionately large heat-losing surface is obvious when conservation of energy is considered. Many rodents are deep hibernators, including ground squirrels, woodchucks, dormice, and hamster. The fat-tailed and mouse lemurs are primates that hibernate or estivate. Among birds, the poorwill (Phalaenoptilus) and some hummingbirds and swifts undergo a lowering of body temperature and metabolic rate in cold periods.

With all deep hibernators, except the bats, hibernation is seasonal, usually occurring during the cold winter months. In all cases, it occurs in animals which would face extremely difficult conditions if they had to remain active and search for food. During a preparation period for hibernation, the animals either become fat, like the woodchuck, or store food in their winter quarters, like the chipmunk and hamster. Prior to hibernation, there is a general involution of the endocrine glands, but at least part of this occurs soon after the breeding season and is not directly concerned with hibernation. Animals such as ground squirrels become more torpid during the fall, even when kept in a warm environment, indicating a profound metabolic change which may be controlled by the endocrine glands. In most hibernators lack of food has little if any effect, and the stimulus for hibernation is not known. It has been reported that an extract from the blood of an animal in hibernation will induce hibernation when infused into an active potential hibernator, indicating that the factor which produces hibernation may be bloodborne.

Hibernation in mammals is not caused by an inability to remain warm when exposed to cold, for hibernators are capable of very high metabolic rates and sometimes do not enter hibernation if exposed to cold for months at a time. When the animal is entering hibernation, heart rate and oxygen consumption decline before body temperature, indicating that the animal is actively damping its heat-generating mechanisms. The autonomic nervous system is involved in this process. As normal hibernation deepens, the heart rate, blood pressure, metabolic rate, and body temperature slowly drop, but in some animals periodic bouts of shivering and increased oxygen consumption occur, elevating the body temperature temporarily and causing a stepwise entrance into hibernation. See Autonomic nervous system In deep hibernation at a steady state the body temperature is 33–35.5°F (0.5–2°C) above that of the environment, and it is a peculiarity of hibernators that the vital processes can function at lower temperatures than those of nonhibernators. The heart rate varies between 3 and 15 beats per minute.

The metabolic rate is less than one-thirtieth of the warm-blooded rate at rest, and the main source of energy is fat. In spite of its low body temperature, the hibernating animal retains a remarkably rigid control of its internal environment. If the environmental temperature drops to 32°F (0°C), the hibernating animal may respond either by increasing its metabolic rate and remaining in hibernation or by a complete arousal from the hibernating state. A hibernating mammal reduces its metabolic rate by nearly 30-fold and shifts from glycogen to lipid (that is, fat stores) as the major fuel source for metabolism. The magnitude of metabolic rate reduction is far in excess of what would be expected solely as a result of a hibernator's lowered body temperature. Moreover, suppression of glycogen metabolism during hibernation must be poised for regular and rapid relaxation during periods of arousal (which are fueled by glycolysis) as well as at the end of the hibernation period. Mechanisms controlling these aspects of hibernation metabolism appear to be the relative acidification of the intracellular fluids of the hibernator.

This is a consequence of the hibernator's tendency to continuously regulate its blood pH (at about pH 7.4, termed pH stat), and of the adoption of a modified breathing pattern that, although variable among species, is typified by periods of apnea lasting up to 2 h that are interspersed between 3–30 min intervals of rapid ventilation. The hibernator is capable of waking at any time, using self-generated heat, and this characteristic clearly separates the hibernating state from any condition of induced hypothermia. During the total period of hibernation, the hibernator spontaneously wakes from time to time, usually at least once a week. In the period of wakefulness the stored food is evidently eaten, but animals which do not store food rely on their fat for the extra energy during the whole winter. The cause of the periodic arousals has not been definitely determined, but it is theorized that the arousal is due to the effect of the accumulation of a metabolite or other substance which can be neutralized only in the warm-blooded state.

As in hibernating endotherms (birds and mammals), a key factor regulating seasonal torpor in ectotherms is the continuous internal monitoring of environmental cues, such as day length, which in turn triggers temporally precise seasonally adaptive changes in systemic function, metabolism, and behavior. A second important factor is the presence in ectotherms of a bioenergetic metabolic system that, when compared to mammals and birds, operates at a much lower intensity and has less absolute dependence on molecular oxygen. The metabolic energy adaptations for seasonal torpor in ectothermic vertebrates are to a large extent similar to those required by vigorious activity or prolonged diving, and thus involve the processing or storage of intermediate metabolitics such as lactic acid, the regulation of intra- and extracellular pH, and enduring periods without access to oxygen. See Energy metabolism , Metabolism hibernation dormancy - Dormancy, hibernation, and estivation in warm-blooded vertebrates | biology | Britannica.com diapause Homoiotherms and heterotherms The term hibernation is often loosely used to denote any state of sustained torpor, inactivity, or dormancy that an organism might exhibit.

Properly speaking, however, use of the term should be confined solely to warm-blooded homoiotherms—i.e., birds and mammals whose feathers or fur serve as insulation to reduce heat radiating from the body and aid in the maintenance of constant body temperatures, which normally are independent of those of the environment. Because warm-bloodedness gives animals an internal physiological stability, they are less dependent on many environmental restrictions, particularly those limitations imposed on organisms by ambient temperatures. For example, only two species of reptiles are found north of the Arctic Circle, but great numbers of birds live and breed there. Warm-bloodedness also signifies a high metabolic rate, a factor that undoubtedly influences normal learning, which depends heavily on the frequency and recency of experiences. Because periods of lowered metabolism interrupt continuous learning experiences, they may explain in part why birds and mammals are so much easier to train than any other animal. The benefits of warm-bloodedness require the expenditure of large amounts of energy through the year and make a heavy demand on available food supplies. The term hibernation is also used to delineate the dormant state only during winter.

In arid regions a reverse phenomenon is seen in which the animal becomes torpid during the hot, dry, barren summer; such hibernation is called estivation . As a means of avoiding environmental stresses, hibernation and estivation are not common devices among warm-blooded animals and they are far less common among birds than among mammals. Some warm-blooded organisms exhibit thermic instability, a heterothermous condition that allows their metabolic rate to be reduced, with a commensurate reduction in body temperature. Heterothermy is a transitional state between cold-bloodedness and warm-bloodedness; the animal is awake and moving during its temperature fluctuations. The body temperature, although not as constant as in humans, is not so low as to force the organism into deep hibernation. Among mammals, two monotremes, the spiny anteater and the duckbill platypus, are thermally unstable; many of the marsupials, including the opossum, the pouched mouse, and the native cat (a weasel-like spotted marsupial of the family Dasyuridae), are also unable to maintain a fixed body temperature.

Britannica Stories EU Considers Rules For Robots The true hibernator not only possesses adaptations that enable it to respond as a homoiothermous animal during certain periods of the year but can also adapt to stressful environmental situations and become essentially a poikilothermous animal during other periods. An animal exposed to food shortages, low temperatures, or lack of water, for example, may “turn off its thermostat” and hibernate until the environment becomes more favourable. Unlike poikilotherms, however, hibernators still retain a measure of temperature control and can change their metabolic levels as required. They can arouse themselves to full activity, whatever the environmental temperature, whereas the arousal of a poikilotherm is dependent upon increased environmental temperatures. During the period prior to hibernation an animal must make a considerable number of gradual physiological and metabolic adjustments that appear to be correlated with temperature, light, and the availability of food. No one set of conditions applies equally to all hibernators: some store food, others do not; some become excessively fat, others gain a more moderate amount of weight.

Generally, as the season advances and as the hibernator becomes progressively more prepared for hibernation, there is an increase of fat deposition and a general readjustment of body temperature, metabolism, and heart rate to lowered levels of activity. Although no single factor or condition can be said to determine when an animal will go into hibernation, specific changes include an increase in the quantity of magnesium in the blood and a reduction in the activity of endocrine glands, such as the pituitary, thyroid, and adrenals. A reduction in gonadal activity has also been observed; hibernation does not occur when the gonads are in an actively functional state. Perpetuation of the species requires that the animal be warm and active during the mating and pregnancy periods. Connect with Britannica Facebook Twitter YouTube Instagram Pinterest There appears to be a relationship between sleep and hibernation; available evidence suggests that hibernation is entered into from a state of sleep. If hibernation is to be considered a form of sleep, then it must be considered a remarkably complex one. Hibernation and sleep are somewhat similar in that essential body processes continue during both periods at a lowered level.

In sleep, the heart beats less rapidly, and breathing is slower; the body produces less heat, necessitating that a sleeping person be protected from the cold. Hibernation in birds Lady Flora Hastings The behaviour of the hummingbird can be contrasted to that of a larger bird, such as the poorwill, which is a nocturnal, insect-catching bird. During an average 24-hour day, the poorwill has brief periods of activity at dusk and just before dawn, the total of which is scarcely more than an hour. The temperature of the poorwill during these periods of activity, which are correlated with the bird’s feeding habits, is between 40.5 and 43.1 °C (104.9 and 109.6 °F). Between periods of activity, the bird rests quietly, and its body temperature drops 1 to 3 °C (2 to 5 °F). During periods when a supply of flying insects is not available, the bird hibernates in depressions in rocks or other suitably protected places, to which it returns each year.

When hibernating, the bird’s temperature is frequently within 1 °C (2 °F) of that of the environment; as a result, the energy saved is great. A poorwill whose body temperature is 5 °C (41 °F) has a metabolic rate only 3 percent of its metabolic rate at normal body temperature. Because the poorwill is a larger bird than a hummingbird, it may take more than an hour for it to emerge from hibernation. Types of hibernation in mammals It takes longer for larger animals than for smaller ones to go into hibernation because heat must radiate from the body before the temperature can be lowered. Thus, it would require a considerable amount of time for large birds or mammals to go into and emerge from hibernation each day, as do bats and hummingbirds. A 200-kilogram (440-pound) bear , for example, would need 5,100 calories to warm from 10 to 37 °C (50 to 99 °F).

Unlike the hummingbird, which uses only 1/85 of its total daily energy expenditure to emerge from hibernation, the amount expended by a bear would be equivalent to its full 24-hour energy budget. Even if there were enough time in 24 hours for a large animal to enter into and emerge from dormancy, therefore, it would be metabolically extravagant, thus defeating a purpose of hibernation. Actually, the most common misapplication of the term hibernation is in relation to the bear, which is not a true hibernator. Its body temperature, which normally averages 38 °C (100 °F), drops during its winter lethargy to about 34 °C (93 °F), seldom getting below 31.2 °C (88.2 °F). Hence, a bear’s temperature during the winter does not approximate that of the environment. This is indicative of winter rest rather than true hibernation. During this inactive period, the bear sleeps but is, nonetheless, warm and capable of activity when stimulated, unlike a true hibernator.

Moreover, it is also during this period when females give birth to cubs that suckle and are maintained by maternal warmth until they emerge from the den in the spring. This behaviour is in contrast with that of the Arctic ground squirrel, whose normal temperature is the same as that of the bear but whose temperature during hibernation drops to near freezing and, in some cases, to a degree or two below 0 °C (32 °F). Although certain mammals are said to hibernate, they do not necessarily enter a state of deep hibernation during winter. Instead, for weeks at a time they may be inactive and lethargic in behaviour, with a slightly depressed body temperature. The chipmunk (Eutamias) is an example of what has been termed a shallow hibernator, as are bears and raccoons. Superficial hibernation, apparently a compromise between the minimum energy requirements of a deep hibernator and the high energy expended by an animal that remains active during the winter, saves energy without the stress of hibernation. The animal can thus conserve food, while still being able to escape from predators and such dangers as flooding of its burrow.

The main energy source during the winter in this shallow hibernation state is food stored in the winter nest. There are instances, however, of shallow hibernators, such as the chipmunk, that enter a state of deep hibernation, particularly if without food. True mammalian hibernation Omitting the thermally unstable mammals, the true mammalian hibernators are those whose lowered body temperatures approximate that of the environment and those who require extensive and complex physiological changes to turn from a warm-blooded animal into an essentially cold-blooded one for an appreciable length of time. Only three orders of placental mammals display such behaviour: the Erinaceomorpha, as exemplified by the hedgehog; the Chiroptera, the bats; and the Rodentia, including the marmot, hamster, dormouse, hazel mouse, and ground squirrel. A typical mammalian hibernator, such as the Arctic ground squirrel, finds a protected environmental niche—in this case, a burrow beneath the surface—and builds a nest of grass, hair, and other materials to provide still further insulation.

The usual hibernating position is one of being curled up in a ball with the extremities tucked tightly against the body so there is a minimal surface-to-volume ratio. After the temperature of the animal has dropped near that of the ambient temperature, it appears to be dead: its respiration is imperceptible, about three irregular breaths per minute; it does not react to outside stimuli in an observable fashion; nor does it react to being handled and uncurled, although such handling will trigger wakening mechanisms. The internal organs, such as the digestive tract and the endocrine glands, are almost totally inactive. Because the process of hibernation necessitates the mobilization of all body resources, it places great demands on the tissues, all of which are directed toward the problem of maintaining the animal’s metabolism at the minimal level necessary for life during the hibernating period. This means that all activity not immediately germane to the process of living at the lowest possible metabolic level ceases. Even bones and teeth deteriorate during hibernation. The hibernator apparently is balanced on a very narrow line between the maintenance of life at a level that makes recovery from hibernation possible and a reduction of metabolism to a level that will lead to death.

Evidence obtained from tissues indicates that the process of hibernation is a precarious method of survival at best and one from which many animals do not awaken. As a mechanism of species survival, hibernation seems effective; for the survival of the individual, however, it is an uncertain and dangerous process. The hibernator does not remain in a continuous state of hibernation from the time it enters in the fall until it emerges in the spring. Hibernating Arctic ground squirrels, for example, awaken at intervals of every three weeks or less. During this time the animals may move about and sometimes emerge from the burrow. These periods of arousal are more frequent at the beginning and end of a hibernation period than in mid-hibernation; and the lower the temperature at which an animal hibernates, the fewer the awakenings. During the period of hibernation about 40 percent of the total body weight is lost, an average of about 0.2–0.3 percent per day. One period of arousal and wakefulness consumes more heat and energy than many days in hibernation.

About 90 percent of the total heat production and weight loss during hibernation takes place during the arousal periods; only 10 percent is required to maintain the animal in hibernation. Thus, in the case of an unusually long or hard winter, the animal may be called upon to use all of its available energy sources in periodic arousals; it then enters one final hibernation period from which it does not awaken. Animals that store food in the nest have a chance to renew their energy requirements by eating when they awaken periodically. Entrance into hibernation Hibernating mammals can be divided into four major groups according to the way they enter hibernation. One group is exemplified by the golden hamster ; it waits a variable time of from one to three months in the cold and then enters hibernation in one major temperature reduction. This is accomplished when the biochemical and physiological preparations have been sufficient to lower the animal to a level at which it is receptive to the hibernating stimulus, which causes the abandonment of the temperature differential between ambient and body temperatures.

A second group, of which the pocket mouse (Perognathus) is an example, prepares for hibernation relatively rapidly, waiting only a few days before becoming torpid in one major temperature decline. The third group, which constitutes most of the mammalian hibernators, includes ground squirrels and marmots . These animals wait only a few days before entering hibernation and then go through a series of steps of torpor and arousal, each one at successively lower body temperatures, until the level dictated by the stage of preparation for hibernation is reached. The fourth group, which includes most of the bats , becomes inactive in the poikilothermous manner; that is, the body temperature follows the ambient temperature. Even though the bat seems ready to hibernate at any season, survival during hibernation depends upon more adequate preparation than is necessary for the transitory periods of torpor. Bats not only exhibit true hibernation during the winter but also have natural periods of hypothermia (subnormal temperature), which are unrelated to hibernation, during the remainder of the year.

The woodchuck , the dormouse, and the California ground squirrel enter hibernation in successive stages, with a complete or nearly complete awakening between each one. In the woodchuck, an initial decline in temperature is followed by an arousal. During the second decline there is a lower and more pronounced fall in body temperature, followed by a less pronounced rise. This process continues until the body temperature is essentially the same as that of the environment. Physiological changes during mammalian hibernation Heart rate and circulation The body temperature of a hibernating mammal is affected by changes in respiration, heart rate, and oxygen consumption; all are apparently mediated by a part of the nervous system. The heart rate decreases prior to a decline in body temperature. In the woodchuck, the rate may drop from 153 to 68 heartbeats per minute within 30 minutes. In the California ground squirrel, the heart may beat as slowly as once a minute at 5 °C (41 °F). In contrast, the hearts of non-hibernators generally will not beat at all at temperatures below 10–20 °C (50–70 °F).

As an Arctic ground squirrel prepares for hibernation, its heart rate and its blood pressure decrease. Both may be detected before a decrease in body temperature can be noted. When the animal enters hibernation, temperatures of both the heart and abdominal regions are identical, indicating an even blood flow between the anterior (front) and posterior (rear) parts of the body. As the body temperature drops, the resistance to blood flow in the peripheral parts of the circulatory system increases because of the increased viscosity (resistance to flow) of the chilled blood and the constriction of the distal arterioles (small arteries) of the body. This peripheral resistance maintains blood pressure at relatively high levels in the deeply hibernating squirrel, even when the heart beats only three or four times a minute. Neural changes The nervous system of hibernators also is acclimated; certain specific structures and pathways are seemingly maintained to regulate and coordinate metabolism as temperatures drop. This adaptation of the nervous system enables changes in the environment to be perceived, even when the animal is torpid.

In the Arctic ground squirrel, measurements of the general electrical activity of the brain indicate a 90 percent reduction when the animal is in hibernation, at which time brain temperatures approximate 6 °C (43 °F). During hibernation, both the peripheral nervous system (all the nerves outside the brain and spinal cord, which constitute the central nervous system) and the spinal cord have an increased sensitivity to certain stimuli; in addition, the areas of the brain that regulate temperature as well as cardiac (heart) and respiratory function remain active at ambient temperatures, below which the mammalian nervous system normally ceases to function. Changes in the circulatory system involving constriction (narrowing) of posterior vessels and the favouring of anterior circulation allow the brain temperature of hibernators to remain a few degrees warmer than the environmental level. This enables the temperature of the brain to remain constant despite fluctuations in the temperature of the skin. Endocrine activity The male sex hormone testosterone stimulates reproductive activity. The golden hamster will not hibernate if injected with more than five milligrams of a hormonal preparation. Hibernation is also prevented if the animal is fed or injected with thyroid hormones or thyroid-stimulating extracts.

The latter would seem to implicate the thyroid as another endocrine gland that plays an important role in hibernation. There is, in fact, a seasonal progression and regression of thyroid activity in hibernators; maximal activity occurs in the spring and minimal activity in the fall. And because hibernation does not take place in the absence of the adrenal glands, it appears that a minimal adrenal activity is also necessary for hiberation and survival. The importance of timing in the annual rhythm of activity and dormancy can be demonstrated: when hibernators are exposed to cold temperatures in spring and summer, they react as do all homoiotherms by increasing their thyroid activity and metabolic rate to maintain normal body temperature. But if they are exposed to cold temperatures in the fall, the thyroid activity and metabolic rate of hibernators are lowered. In some species, a combination of decreased food and lower ambient temperature is required to reduce activity of the thyroid gland and to produce hibernation, although cold alone is sufficient in ground squirrels and the dormouse. Although hibernation does not take place during periods of gonadal activity or stimulated thyroid activity, it can occur during increased activity of the pituitary gland .

This would suggest that there is a dissociation of cellular growth and hormone synthesis that is normally controlled by hormone secretion of the pituitary and its target organs. Thus, the triggering mechanism for the resumption of normal endocrine activity apparently resides elsewhere than in the pituitary. The function of the hypothalamic region of the brain in regulating appetite, fat deposition, water intake, and diuresis (increased excretion of urine), as well as in the control of temperature and sleep, would appear to make it a key area in directing life processes of the hibernator. Furthermore, the fact that the hypothalamus regulates the pituitary and other endocrine glands not only supports this thesis but also indicates that this area of the brain is the prime, or master, regulator of the entire hibernation process. Reproductive cycles The Arctic ground squirrel may spend more than half its life in hibernation. It thus must be able to breed, rear young, maintain its home burrow, and prepare for the period of hibernation during an activity period of less than six months. This requires considerable adaptation of both metabolic and behavioral patterns.

Prior to entering hibernation in late September or early October, there is a renewal of sexual activity in the testes of males, and, throughout the period of hibernation, they continue to grow. On the Arctic slope in early May, the male ground squirrel emerges from its burrow. As it utilizes the remaining fat and eats the stores of seeds and other food still in the nest, the male reaches a period of reproductive readiness. Mating takes place in the middle of May, and the young are born in the middle of June, after a gestation period of about 25 days. By the middle of July the young are above ground and eating the green Arctic vegetation, which they continue to eat until the onset of hibernation. By October, both the young of the year and the adults from the previous year weigh nearly 1,000 grams (2.2 pounds). In the bat, the reproductive cycle is interrupted by hibernation. Gonadal activity in the male reaches its maximum in the fall, when copulation with the female occurs. The animals then hibernate, and the production of sperm in the male ceases.

The sperm deposited in the female are stored in her reproductive tract throughout the period of hibernation; fertilization occurs the next spring, when the eggs are ovulated (released from the ovaries) within a few days after awakening from hibernation. The only exception to the general hibernation–reproduction pattern of bats is the vespertilionid bat ( Miniopterus), in which there is no delayed ovulation and fertilization . In this species the eggs are ovulated soon after copulation, in the fall, and fertilized immediately. During the ensuing period of hibernation embryonic development is initiated and slowed, but it does not actually cease. The young are born in the early summer, soon after hibernation ends. The introduction of hibernation during pregnancy makes the gestation period several months longer than in non-hibernating tropical members of the same genus. Cyclical reproductive activity has thus become adapted to the shortened activity season available to the hibernator. But although the annual sequence of reproductive events is known, the external stimuli that regulate the reproductive cycles of bats and other hibernators are not known. More knowledge is needed concerning the endocrine and nervous mechanisms that presumably regulate reproductive processes internally.

It has been suggested that the pituitary–gonadal relationship influences the hibernating cycles as well as the reproductive cycle, hence both the latter and homoiothermism are controlled by a common mechanism. Such a suggestion is attractive in that the mechanism solves the regulation problems, but more needs to be known of the way in which hibernation directly or indirectly modifies the action of endocrine and neural mechanisms that direct the reproductive cycle. Protection from disease and radiation Hibernating organisms have a certain degree of resistance to infectious diseases that appears to be attributable to at least three factors, all of which are related to temperature. One is the fact that the lowered temperature of the host and the commensurate slowing of its metabolic processes prevent the multiplication of parasites to a greater extent than they affect the host’s defensive mechanisms. Second, lower temperatures are more harmful to the development of a disease organism than to the host, as has been shown with the parasite Trichinella spiralis. In bats hibernating at 5 °C (41 °F), only larvae have been recovered from the intestines; but mature adult worms have been recovered from the intestines of bats kept at 35 °C (95 °F).

The third factor is that the influence of low temperature on the chemical composition of the host tissues may also affect infectious organisms. Hibernation also seems to protect animals from radiation . When ground squirrels are irradiated with radioactive cobalt while hibernating, they are found to be more resistant to the effects of the radiation than are squirrels irradiated while warm and active. This resistance, which is apparent over a wide range of doses, suggests that protective mechanisms function in the hibernating animal. In both hibernating and non-hibernating animals, repair processes within cells occur the first day after irradiation; however, when the metabolic requirements of cells are small, as in hibernation, the injured cells seem to be more capable of repair, and survival is greater. The large metabolic requirements imposed on injured cells of warm and active animals appear to render them incapable of an adequate repair response. Awakening from hibernation The process of awakening in the Arctic ground squirrel takes about three hours. There is a rapid rise in heartbeat and a decrease in peripheral circulatory resistance; the area around the head and heart warms more rapidly than the posterior part of the animal.

This differential vasodilatation (widening of the blood vessels) in the anterior part of the body is a unique and vital part of the awakening process. The concentration of active circulation in this region results in a high blood pressure and an efficient and rapid warming. If a drug is administered during awakening that causes vasodilatation throughout the body, there is a marked drop in blood pressure even though the heart may almost double its rate; thus, the heart cannot maintain a high blood pressure at this time if all blood vessels are dilated. Later during the arousal process, after the anterior part of the body has been warmed, the posterior part of the animal warms rapidly. Despite the deterioration of glands and tissues and the drastic reduction of all metabolic activity during hibernation, within 24 hours after arousal, all the squirrel’s physiological processes are essentially normal. This rapid repair and recovery mechanism is one that requires further study. h2g2 - Dormancy in Animals - Edited Entry Dormancy in Animals Content from the guide to life, the universe and everything Dormancy in Animals 2 Conversations Organisms everywhere, both plant and animal, are subjected to stress from their environment.

In order to avoid this, some animals migrate vast distances, while others are just stubborn enough to stay. These animals change their behaviour and the habitats 1 that they stay in in order to try and reduce environmental stresses. However, other creatures become dormant. Taking on an inactive state, they have a lower rate of metabolism 2 than normal, which enables them to conserve energy and place few demands on their environment. Most groups of the animal kingdom have species that become dormant. Types of Dormancy There are a number of types of dormancy, and the difference between them is rather woolly, scientifically speaking. Hibernation - Hibernation is often used to describe any winter dormancy in both warm and cold-blooded vertebrates, including bears, shrews and other animals that are inactive over winter. However, there is a difference between dormancy and hibernation. Hibernation involves a huge decrease in body temperature, whereas dormant animals tend to reduce their body temperature and metabolism only slightly. Animals that truly hibernate tend to be small mammals, weighing less than 1kg. Although bears and other animals are said to undergo hibernation, they are able to wake, move around and eat on warmer days.

Some bears are even able to give birth during the winter. Aestivation - Aestivation is dormancy during summer months, rather than winter months. It occurs with a decrease in water supplies and high temperatures. Daily Torpor - This involves a period of dormancy that lasts, not unsurprisingly, less than a day. This is a shallow decrease in metabolism, and usually results from a short food supply. Many species of animals undergo daily torpor. Nocturnal Hypothermia - Many birds display nocturnal hypothermia. The body temperature drops by roughly 3-5°C overnight, and their metabolism also reduces to about half of the daily, active rate. In addition, it should be remembered that the degree of metabolic reduction and the period of time that the organism remains dormant vary from species to species. Some animals undergo a series of short-duration 'deep sleeps', while others experience large periods of time with a severe decrease in their metabolic rate. Why Dormancy? The process of dormancy appears to have evolved independently in a large number of different species, and, as such, there are a wide number of mechanisms by which a period of dormancy can be entered, depending on the physiology of the organism.

In many species, dormancy is an essential part of the life cycle, enabling an organism to change hugely, while undergoing minimal impact. In some cases, it allows organisms to survive huge climatic changes, such as a pond or river drying up. However, the type of dormancy most commonly associated with the word is that encountered when an animal becomes dormant during a period of cold weather. Dormancy, in short, allows organisms to adapt better to their environment during periods of hardship. It enables species to take advantage of certain environmental niches, which would otherwise be impossible to make habitable. Causes of Dormancy Dormancy can be caused by a number of different things. The most important are changes in temperature; slightly less important is the availability of factors such as food, water, oxygen and carbon dioxide. Usually, the inducing factor of dormancy is a slight shift out of the temperature range for the organism. However, as a direct result of the temperature flux, food, water and oxygen are also usually likely to decrease, which reinforces the stimulus to become dormant. As an example, animals in polar regions become dormant during the winter months due to the lack of food.

However, during the summer months in the desert, some animals become dormant to compensate for a lack of water. Other factors that can also stimulate dormancy are light intensity and other environmental factors. Most environmental influences that stimulate dormancy are, not surprisingly, cyclical. There are two cycles that have the greatest effect - circadian 3 and annual. Daily changes in light and temperature induce rhythmical changes in metabolism, whereas annual changes affect the availability of food and water. The other factors, oxygen and carbon dioxide, do not vary cyclically, but are brought about by the organism itself. Strangely enough, it has been scientifically shown that once an organism has become adapted to performing a series of behavioural patterns in response to its environment, it will continue to exhibit these even after the stimulus is removed 4 . Animals often respond in the same way as before if removed from their original habitats. Dormancy in Protozoa and Invertebrates - Cyst Formation Protozoa Many parasitic and free-living protozoa 5 undergo dormancy by excreting a protective cyst. It can be induced by temperature change, pollution, or lack of food or water. Euglena has two types of cysts, both formed in response to stressful conditions.

However, one results in a cyst that can contain up to 32 daughter organisms, resulting from asexual reproduction. These daughter cells then emerge from the cyst once the environmental conditions have been restored to normal. Parasitic protozoa experience cysts in their life cycle. For example, Entamoeba histolytica 6 forms cysts in the intestine of infected individuals which then are released from the body through faeces. Once food or water containing the cysts enters the digestive tract of another person, the amoebas are released from cysts and can infect the new host. If it were not for the creation of cysts, many diseases would be easier to control. However, the cysts enable cells to remain viable (alive) for many weeks. E histolytica cysts can withstand temperatures of around 70°C and some chemicals. Invertebrates Cysts are formed in the life cycle of invertebrate parasites, such as flukes, and are used for the passage from one host to another. However, there is also the formation of cysts during periods of environmental stress. Freshwater sponges and some marine species form gemmules 7 within the body of the organism.

These have a resistant covering, and are released upon the death and disintegration of the sponge. Once conditions have been returned to normal, the cell mass escapes the cover and a new sponge is formed. Rotifers 8 produce winter eggs that have thick, resistant covers that are similar to the protozoan cyst, enabling it to survive for long periods of time. These eggs can survive drought and freezing conditions, and can be dispersed by wind or other animals. This gives the cyst an added advantage - it prevents too many rotifers living in the one area and introduces the creature to new habitats and niches. Land snails are mostly dormant during the day, and do not emerge from their shell until night. With unfavourable conditions, they are able to secrete a membrane of mucus and slime that covers the opening of the shell until things improve. Diapause in Insects Diapause 9 can occur at any stage of the life cycle. It can be recognised in two ways - as a stopping of growth in the immature insect, and as a stopping of sexual activity in the adult insect. Again, it can occur either as a result of environmental conditions, or because it is a part of the life cycle, of which butterflies are a good example.

Many insects are able to survive winter through diapause. Even the adult butterfly or mosquito is able to survive so long as they are in a sheltered spot. Butterflies often remain in shrubbery over the winter, where they can be covered in snow and ice, but still emerge unscathed in spring. Other insects form cocoons, or shelter in already existing hiding places 10 . The start of diapause again depends on the influence of a combination of environmental factors upon the regulatory systems of the insect. Temperature and the length of day influence the endocrine functionality of the insect brain 11 , affecting the release of hormones that influence other organs. The main organs affected are the prothoracic glands. These glands secrete the hormone ecdysone normally: in its absence all growth and metamorphosis is stopped. Once the temperature increases and the days begin to get longer, ecdysone is secreted, enabling the continuation of growth and development. The time at which diapause comes to an end is species-dependent. Often only favourable conditions are required, but in some species, a further stimulus is required. Also, there are instances where a certain amount of time must have passed before a stimulus is effective 12 .

For instance, the eggs of the mosquito Aedes vexans will remain in a state of diapause until the damp soil on which they are placed floods into a pool that is suitable for the larvae. Eggs of A canadensis, another mosquito, are laid in the same soil as A vexans, but will not leave diapause until subjected to a cold spell. This means that although both species lay eggs in early summer, A vexans will hatch in pools formed in the summer rains, while A canadensis will hatch in the pools formed by spring rain, after winter. Dormancy in Cold-Blooded Vertebrates Two kinds of dormancy can be distinguished in vertebrates on the basis of body temperature. Most vertebrates are cold-blooded, or poikilotherms. This term derives from the fact that their body temperature is determined by the temperature of the environment, and is not maintained at a constant temperature by various internal processes or homeostasis. Warm-blooded vertebrates, homoiotherms are able to maintain a constant temperature irrespective of their environmental conditions. Fish and Amphibians Cold-blooded animals' metabolism is mostly influenced by temperature, nutrition and photoperiod 13 .

Photoperiod is extremely important for fish and amphibians. Although fish remain active throughout the year, their activity levels alter depending on the temperature of the surrounding waters. Some species can survive periods of brief, superficial freezing or super-cooling where the body fluids don't actually freeze, as the surrounding temperatures are below the freezing point of internal body fluids. However, fish do not reduce their metabolism to the extent of previously described creatures, but maintain some activity throughout the entire year. As such, they cannot truly be said to become dormant. The other environmental stress that fish can suffer is drought. Lungfish burrow deeply into mud during periods of drought. They cocoon themselves in a layer of slime and become dormant. The gills do not function and an air bladder is used to breathe. Fat reserves provide their energy and urea is excreted rather than ammonia, to preserve water 14 . Amphibians find suitable places to remain dormant in until favourable environmental conditions return. Frogs and salamanders often congregate in large numbers where it is moist. Toads often become dormant in burrows and frogs, during periods of drought, may become dormant in a cocoon of mud.

Effects of Temperature on Reptiles Reptiles depend on external sources of heat to keep warm. As such, they are able to survive low temperatures by finding an environment where the temperature will not fall below freezing. This can be found underground, with the depth depending on the thermal conductivity of the soil and the minimum temperature. It is this dependency that controls where reptiles are found. The Arctic and Antarctic are too cold as the subsoil is permanently frozen. The same is true for neighbouring regions; however, this is compounded with the fact that the length of summer is too short to enable the completion of the life cycle. Snakes are the most robust of reptiles, and have been found at both high altitudes 15 , and latitudes 16 . Low temperatures directly induce dormancy in reptiles. The adder prepares for dormancy at a temperature of 8-10°C, and ends dormancy a few days after the maximum temperature is above 7.5°C. This temperature dependence results in a variation of dormancy period, depending on where the species is found. It can be as long as 275 days in northern Europe, to 105 days in southern Europe.

In the UK, because of the Gulf Stream, it can even be as little as a few weeks. Reptiles also tend to become dormant when the temperature is too high. However, there is a difference in the physiology produced during summer and winter. Animals have a higher tolerance to low temperatures than higher temperatures. There is a wide gap between an animal's normal temperature and the low temperatures that cause death. With high temperatures, this gap is not so wide. This causes reptiles to find cool and shady places, where they can avoid the extreme heat. Although their metabolism doesn't slow, movement is restricted because of their situation. This behaviour can occur daily during the summer in the desert. The water required by a reptile during dormancy is less than normal, and is provided by the metabolism of fat reserves. If a reptile exists in an environment that has dry and wet seasons, the factor of water availability can result in a longer period of dormancy during the dry season than in the wet season. Often a large number of snakes are found dormant in the same den. This is due to the limited niches available. It is possible for other reptiles, such as lizards or toads to also share the same den over winter.

Although snakes often stay in an evacuated burrow, often that of a prairie dog, it is unlikely that snakes will remain there while the original owner remains. Effects of Latitude Latitude affects the period of dormancy and their period of daily activity (due to the difference in daylight). Many snakes are active during the early evening, but in northern countries, such as in Scandinavia, snakes are active during the day and take as much advantage of daylight as possible 17 , in order for their life-cycle development to occur fully. However, because of the shorter summer, growth and development is severely reduced, sexual maturity is delayed and the reproductive period is over two years, rather than the one year experienced in lower latitudes 18 . So, dormancy is a great way for animals to adapt to protect themselves against extreme environmental conditions. It is more energetically sensible than 'toughing it out' over the winter. 1 A habitat is the immediate surrounding of an animal. 2 These are the chemical processes which are responsible for the activity, nourishment and growth of an organism. 3 The circadian cycle is the 24 hour period in a day.

4 This explains why many of us find it impossible to adapt to waking early once we have become accustomed to sleeping in for many years. 5 A protozoan is a single-celled animal. 6 The causative agent of amoebic dysentery. 7 Gemmule is French, derived from the Latin word for 'bud'. 8 Rotifers are microscopic, aquatic animals. 9 A period of reduced metabolic activity. 10 Hence it is possible to have a butterfly lie on the wall of your stairwell for the duration of winter, and then fly off in the spring. 11 The endocrine function of the brain is to release hormones to control the regulatory function of the body. 12 This means that in some species, even though a stimulus to induce the ending of diapause may have occurred, the insect will not increase its metabolism until a certain period of time has passed. 13 'Photoperiod' is the hours of daylight experienced every day. 14 Ammonia is a highly toxic substance, and a large quantity of water is needed to dilute it to a suitable concentration. Urea has a low solubility, and is excreted in preference to ammonia.

Other animals which need to preserve water also excrete urea, such as desert animals. 15 The Himalayan pit viper exists at an altitude of 5,000 meters (16,000 feet). 16 Some species of adder can be found in the Arctic circle. 17 This occurs as a result of the length of summer being smaller than that of more equatorial countries. 18 Young are produced once every two years, rather than once a year. Christmas in Yellowstone | Animals That Hibernate | Nature | PBS Christmas in Yellowstone Explore more from this episode More A precise, fixed definition of hibernation is somewhat elusive. A common way of explaining hibernation is to call it a state of winter dormancy, a period of inactivity in which an animal will conserve energy by maintaining a lowered body temperature for most of the winter. Yet there are also creatures, such as the North American desert tortoise, that employ a comparable state of dormancy and body temperature regulation to protect themselves against extreme heat. This may be called aestivation. There is another term, however, brumation, that refers to a winter state of sluggishness in certain reptiles and amphibians, which don’t maintain a high body temperature.

Body temperature, then, may be somewhat misleading as a telltale indicator of hibernation. Perhaps the best way to look at hibernation, given the circumstances, is as a set of seasonal adaptations that animals employ to survive regularly occurring periods of famine. Here is a closer look at the adaptations of five animals that face the challenges of a winter climate: Bats Northern bat species overwinter in caves where they can maintain a lowered body temperature that will allow them to pass the winter months, when their prey will disappear, without feeding. The choice of a cave is paramount: too warm, and the bats’ level of activity will rise, and they’ll starve; too cold and the bats will freeze or exhaust themselves, shivering to keep warm. To find the right cave, the bats must migrate — sometimes hundreds of miles. Bats from the north may fly south, while bats from the south may fly north. With experience, the bats learn which caves are best suited to winter survival — and they will return to a good cave year after year, thousands upon thousands of bats in a single cave. Bears Bears are often thought of as a very typical hibernating animal when, in fact, their winter behavior is quite extraordinary.

In the late summer and early autumn, bears will begin to gorge on food, eating five times their usual diet to accumulate a five-inch layer of fat that will sustain them the entire winter. In the late autumn the bears will lose their appetites and seek out a potential den. Black bears make their dens in a hollow tree, under roots, beneath a pile of brush or branches. Grizzlies dig their dens into the side of a hill, covering the floor with branches and grass. Once in its dens den, a bear’s body temperature will never drop very far below normal and, though it becomes groggy, a bear is easily roused by outside disturbances. What’s so unusual is that, in spite of their nearly normal metabolism their metabolism is not nearly normal, but rather quite extraordinary, bears don’t need to drink or urinate all winter long. They survive on the thickness of their fat alone, while nitrogen, a potentially harmful metabolic waste product that humans excrete through urination, is actually recycled into protein — which helps maintain the bears’ muscle tissue, even as they are inactive all winter long! Meanwhile, the bears stay hydrated because water is not lost through urination.

These and other physiological processes — such as how the bears keep their bone strength during these prolonged periods of inactivity — are currently being studied for their potential to help humans offset the negative effects of aging, osteoporosis, space travel, and the lethargy of modern day office work. Chipmunks Chipmunks are a kind of ground squirrel known for their enormous, expanding cheeks, which, each fall, they will fill to capacity with load upon load of seeds. The seeds are carried back to the chipmunk’s underground system of hibernation burrows, where there is a special chamber reserved for foodstuffs. Chipmunks spend most of the winter underground in a state of dormancy, their body temperatures lowered to conserve energy. Yet the chipmunks won’t hibernate all winter. By March they will be awake again for mating season, in spite of the often still-deep snow. It is then that the stash of seeds will prove Deer Mice Deer mice range from the northern tree line in Alaska and Canada south to central Mexico, covering much of North America.

They are a small species, 10 to 24 grams, roughly the size of house mice, yet they do not hibernate — somewhat surprising for a creature so tiny. Instead, they employ a series of adaptations to conserve energy. First, they build deep nests in tree holes, stumps, logs, or even cabins or other outbuildings. They also huddle together, as many as ten or more mice, to conserve heat. Deer mice are nocturnal, so during the day, they may go into torpor, a state of inactivity in which their body temperatures drop. Usually the torpor ends by late afternoon. Altogether, these three adaptations will allow a deer mouse to save up to 2.5 times the energy of a mouse that doesn’t employ these techniques. Wood Frogs Wood frogs can be found farther north than any other reptile or amphibian in North America, from the northeastern United States, across Canada, and into Alaska. In the winter, one of these frogs may be found underneath a pile of leaves, rock hard and cold — frozen, apparently.

The frog isn’t dead, however, but engaged in a physiological process known as frost-tolerance in which as much as 65 percent of its total body water will freeze. The frog survives by not allowing ice to form in the cells themselves, which could slash and permanently damage the membranes. The frost-tolerance process is triggered by the onset of cold weather, when the frog’s fight-or-flight response releases adrenaline into the blood, which in turn, sets off a response in the frog’s liver that converts glycogen to glucose. This glucose is what prevents the actual cells from freezing while special proteins allow only the liquid within the frog to be concentrated together and frozen. Meanwhile, the frog’s breathing and heartbeat will stop — until the spring, when the frog will thaw out and resume breathing and pumping blood again. Although many amphibians survive the winter by digging down into the ground, below the frost line, three other North American frog species employ the wood frog’s method of frost-tolerance: the gray tree frog, spring peeper, and chorus frog. More from Christmas in Yellowstone (10)

Question: What is the state of inactivity through the dry, summer season, as hibernation is the dormancy of the winter months?
Answer: Aestivation